Apache Derby project- http://db.apache.org/derby

Apache Derby: Off to the Races- http://www.amazon.com/exec/obidos/tg/detail/-/0131855255

ibm_db2 extension for PHP- http://php.net/ibm_db2

A Gentle Introduction to SQL (includes SQL92 BNF) - http://sqlzoo.net

Comparison of different SQL implementations - http://troels.arvin.dk/db/rdbms/

SQL in a Nutshell- http://www.oreilly.com/catalog/sqlnut2

SQL API Portability: Mysql-isms To Be Avoided for API Portability- http://home.fnal.gov/~dbox/SQL_API_Portability.html

ADOdb- http://adodb.sourceforge.net

MDB2- http://pear.php.net/mdb2

PEAR::DB- http://pear.php.net/db

PHP Data Objects (PDO)- http://php.net/pdo